Re: [PATCH v3] KVM: nVMX: Fix incorrect preemption timer vmexit in nested guest

[Haozhong Zhang]

On 07/06/16 15:32, Paolo Bonzini wrote:
> 
> 
> On 06/07/2016 15:26, Haozhong Zhang wrote:
> > On 07/06/16 19:42, Wanpeng Li wrote:
> >> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> >>
> >> BUG: unable to handle kernel NULL pointer dereference at           (null)
> >> IP: [<          (null)>]           (null)
> >> PGD 0 
> >> Oops: 0010 [#1] SMP
> >> Call Trace:
> >>  ? kvm_lapic_expired_hv_timer+0x47/0x90 [kvm]
> >>  handle_preemption_timer+0xe/0x20 [kvm_intel]
> >>  vmx_handle_exit+0x169/0x15a0 [kvm_intel]
> >>  ? kvm_arch_vcpu_ioctl_run+0xd5d/0x19d0 [kvm]
> >>  kvm_arch_vcpu_ioctl_run+0xdee/0x19d0 [kvm]
> >>  ? kvm_arch_vcpu_ioctl_run+0xd5d/0x19d0 [kvm]
> >>  ? vcpu_load+0x1c/0x60 [kvm]
> >>  ? kvm_arch_vcpu_load+0x57/0x260 [kvm]
> >>  kvm_vcpu_ioctl+0x2d3/0x7c0 [kvm]
> >>  do_vfs_ioctl+0x96/0x6a0
> >>  ? __fget_light+0x2a/0x90
> >>  SyS_ioctl+0x79/0x90
> >>  do_syscall_64+0x68/0x180
> >>  entry_SYSCALL64_slow_path+0x25/0x25
> >> Code:  Bad RIP value.
> >> RIP  [<          (null)>]           (null)
> >>  RSP <ffff8800b5263c48>
> >> CR2: 0000000000000000
> >> ---[ end trace 9c70c48b1a2bc66e ]---
> >>
> >> This can be reproduced readily by preemption timer enabled on L0 and disabled 
> >> on L1.
> >>
> >> Preemption timer for nested VMX is emulated by hrtimer which is started on L2
> >> entry, stopped on L2 exit and evaluated via the check_nested_events hook. However,
> >> nested_vmx_exit_handled is always return true for preemption timer vmexit, then 
> >> the L1 preemption timer vmexit is captured and be treated as a L2 preemption 
> >> timer vmexit, incurr a nested vmexit dereference NULL pointer.
> >>
> >> This patch fix it by depending on check_nested_events to capture L2 preemption 
> >> timer(emulated hrtimer) expire and nested vmexit.
> >>
> >> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> >> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> >> Cc: Yunhong Jiang <yunhong.jiang@xxxxxxxxx>
> >> Cc: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> >> Cc: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
> >> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> >> ---
> >> v2 -> v3:
> >>  * update patch subject
> >> v1 -> v2:
> >>  * fix typo in patch description
> >>
> >>  arch/x86/kvm/vmx.c | 2 ++
> >>  1 file changed, 2 insertions(+)
> >>
> >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> >> index 85e2f0a..29c16a8 100644
> >> --- a/arch/x86/kvm/vmx.c
> >> +++ b/arch/x86/kvm/vmx.c
> >> @@ -8041,6 +8041,8 @@ static bool nested_vmx_exit_handled(struct kvm_vcpu *vcpu)
> >>  		return nested_cpu_has2(vmcs12, SECONDARY_EXEC_XSAVES);
> >>  	case EXIT_REASON_PCOMMIT:
> >>  		return nested_cpu_has2(vmcs12, SECONDARY_EXEC_PCOMMIT);
> >> +	case EXIT_REASON_PREEMPTION_TIMER:
> >> +		return false;
> >>  	default:
> >>  		return true;
> >>  	}
> >> -- 
> >> 1.9.1
> >>
> > 
> > This patch also fixed the crash of L1 Xen with L2 HVM guest. Xen does
> > not enable preemption timer for HVM guests, and will get panic if it
> > receives a preemption timer vmexit.
> 
> Thanks!  I'm still not sure why the bit is set in the vmcs02 though...
> 

Yes, it looks really weird.

I replaced "return false" in Wanpeng's patch by

    pr_info("VMCS: preemption timer enabled = %d\n",
            !!(vmcs_read32(PIN_BASED_VM_EXEC_CONTROL) & PIN_BASED_VMX_PREEMPTION_TIMER));

and redid my test. As expected, L1 Xen crashed due to the unexpected
preemption timer vmexit. I got a log from above statement just before crash:

    VMCS: preemption timer enabled = 1

which is expected to be 0, because preemption timer is disabled in
vmcs02. I also modified L1 Xen to dump VMCS at crash, and it says
preemption timer is disabled.

I noticed Jim Mattson recently sent a patch "KVM: nVMX: Fix memory
corruption when using VMCS shadowing" to fix the inconsistency between
vmcs12 and its shadow. Is it relevant here? I'll test his patch
tomorrow.

Thanks,
Haozhong